This invention relates to a hardened bearing surface and method of forming, and particularly to such a hardened bearing surface on a drill bit using rotary cutters.
It is desirable under certain conditions to have a bearing surface formed of a material harder than the remainder of the base material on which the bearing surface is formed. It is necessary at times to perform other machining or finishing operations on the base material, such as the drilling of openings, cutting of grooves, or welding, and for these purposes, it is desirable that the properties of the base material be suitable for welding and machining. For example, it is difficult to weld materials with a carbon content above about 0.35 percent and it is more difficult to machine materials when the hardness is over a Rockwell C (RC) hardness of about forty (40). For a bearing surface, however, it is desirable to have an RC hardness of around fifty-five (55) or sixty (60), to improve the wear characteristics of the bearing, thereby prolonging the life and minimizing replacement of such bearing. A localized hardening of the base material at those areas where bearing surfaces are provided provides the necessary hardness. Other desirable properties in the bearing areas include a so-called high hot hardness which permits the bearing to retain its hardness at high temperatures, such as around 500.degree. C., for example.
For example, U.S. Pat. No. 4,303,137 dated Dec. 1, 1981, shows a method for making a cutter for a rock bit in which the ball race in the cutter is hardened to obtain a Rockwell C hardness of around fifty-five (55) to sixty (60). This is accomplished by applying energy to the surface of the ball race by induction heating, an electron beam, or a laser beam to austenitize a surface layer which is rapidly cooled for hardening.
Another arrangement for forming a hardened bearing structure is illustrated in U.S. Pat. No. 4,474,861 dated Oct. 2, 1984 in which a bearing structure is provided having a hard-facing metal with intervening soft metal areas to form a composite bearing surface. The hard-facing metal is metallurgically bonded to the underlying base metal. The fusion or metallurgical bonding of the hard-facing metal to the base metal is provided by a high intensity energy beam, such as a laser beam, to effect fusion of the two metal compositions or materials. The composite bearing surface formed has spaces between the hard metal filled with a melt of a soft metal which has a Rockwell C hardness of at least five (5) units less than the hardness of the hard metal.
U.S. Pat. No. 4,243,867 dated Jan. 6, 1981 illustrates an apparatus for fusibly bonding a powder coating material to a base material utilizing a laser beam to melt the powder material thereby to form a fusible bond of coating material on the surface of the base material. With the coating material illustrated, it is possible for the bearing surface to have properties substantially different from the substrate. However, the bond between the coating material and the base material is a limiting factor for some applications such as a lug journal ball bearing race in a rotary cutter drill bit.